How Aquatic Sloths Adapted to Their New Life in the Sea

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How Aquatic Sloths Adapted to Their New Life in the Sea

Amson et al., PRSB

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For four million years, Peru’s ocean shores hosted several species of large aquatic sloths. It goes without saying that we wish these creatures were still around. But they’ve been extinct for about four million years.

Luckily, there are lots of sloth fossils left at a site in Peru known as the Pisco Formation, roughly 500 kilometers south of Lima. Here, the fossils tell the story of critters that gradually became more aquatic – in essence, reversing the evolutionary trend that saw our distant ancestors emerge from the sea and colonize the land.

Recently, a team of scientists surveyed Peruvian sloth fossils belonging to five different species of Thalassocnus. The animals lived from 8 to 4 million years ago. The team determined that these fossils show clear signs of one bony adaptation you might expect to see in animals returning to the sea: Later sloths have denser bones, a trait that would have made them less buoyant and more able to get down to the seafloor to snack on plants without wasting a lot of energy.

The study is the first to link adaptations in the sloths' bones with their return to the sea, says Eli Amson, a graduate student at the Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements at the Sorbonne University. Amson and his colleagues reported the results of their bony analysis March 12 in Proceedings of the Royal Society B.

To study the sloth bones, the team first retrieved some samples from fossils kept at museums in Lima, Peru and Paris, France. Then they used CT-scans or thin slices of bone to measure the amount of bone compactness, which is closely related to density. Technically, it's the ratio of solid bone to all types of bone, including bits with cavities. More compact generally means more dense.

They measured compactness in the ribs, femurs, tibia and other long bones, in fossils spanning the 4-million year period of history. While all the fossils indicated a degree of compactness suggesting at least a partial aquatic habitat, the team found that fossils from the later time points, like Thalassocnus yaucensis had more compact bones than those from earlier time points, like Thalassocnus antiquus.

“The ribs and all the limb long bones of Thalassocnus were extremely dense,” Amson says. “Additionally, the late species features a greater degree of compactness, the series of 5 species giving in fact a detailed documentation of the gradual acquisition of this adaptation.”

Amson also points out that the data suggest such adaptations can occur over a period of only several million years – an evolutionary eyeblink.

The authors present a compelling account of how the sloths returned to the sea, says Olivier Lambert, a vertebrate paleontologist at the Royal Belgian Institute of Natural Sciences. “There is a clear sequence of succeeding species for a single lineage, and from a single geographic area,” said Lambert, who was not involved in the work.

Prior studies had suggested similar trends in sirenians – manatees and dugongs – as well as whales. Now, Lambert says, he wonders what other kinds of adaptations the sloths might display, and if there might be differences in limb proportions or how the muscles attach to the bones.